1. Field of the Invention
The invention relates to suspension inductor devices; and in particular to suspension inductor devices with high inductance.
2. Description of the Related Art
For high frequency design applications, both DC and high frequency signals play equally important roles. DC signals can provide an operational active circuit within a typical working frequency range such that it can deal with transmission of high frequency signals such as amplified signals, and can reduce noise index and conduct high power transmissions. Meanwhile, the active circuit can transmit data with high frequencies. Theoretically, the DC signal and high frequency are operationally independent with each other. In practice, however, the DC signal levels are often shifted due to high frequency signal perturbations such that the operational active circuit cannot work within the typical working frequency ranges. Moreover, the DC signal always introduces various noises such that the high frequency signal is mixed with undesired additional noises resulting in demodulation failure by communication systems.
Generally, the equivalent impedance of an inductor increases as frequency rises, which can be indicated by Eq. 1:Z=jwL w=2×π×freq L=inductance   Eq. 1
The equivalent impedance of an inductor, therefore, will become very large at high frequency blocking transmissions of signal. Since the DC signal theoretically does not have frequency and its equivalent impedance is very small, the DC signal can successfully pass through the inductor. As a result, the inductor can function as a separator, separating the DC and high frequency signals to ensure the circuit system operates normally. Additionally, when designing a relatively lower frequency (˜MHz) circuit, inductors with high inductance are needed to achieve high impedance due to its relatively lower operational frequency. Alternatively, when designing a high power circuit, inductors with high inductance are needed to block out high frequency signals preventing signal leakage to the current terminal. Inductors with high inductance are thus indispensable in circuit design and application.
Conventional inductor devices, however, require a larger layout area to fulfill high inductance effects, while a larger layout area causes undesirable signal losses. For example, the characteristic equivalent impedance model for transmission lines can be indicted by Eq. 2:
                              Z          0                =                              120            ⁢            π                                                              ɛ                e                                      ⁡                          [                                                W                  d                                +                                                      1.39                    .                                          +                      0.667                                                        ⁢                                      ln                    ⁡                                          (                                              W                        d                                            )                                                                      +                1.444                            ]                                                          Eq        .                                  ⁢        2            
If inductor devices with higher impedance or higher inductance are desirably achieved, a thicker substrate or thinner transmission lines are required. Alternatively, coupling capability of the inductor coil has to be improved, as indicated by Eq. 3:
                              L          21                =                                                            μ                0                                            4                ⁢                π                                      ⁢                                          ∮                                  S                  ⁢                                                                          ⁢                  1                                            ⁢                                                ∮                                      S                    ⁢                                                                                  ⁢                    2                                                  ⁢                                                                                                                              ⅆ                          ℓ                                                ->                                            1                                        ·                                                                                            ⅆ                          ℓ                                                ->                                            2                                                        R                                                              =                                    L              12                        ⁢                                                  ⁢                          μ              0                        ⁢                          :                        ⁢                                                  ⁢            Air            ⁢                                                  ⁢            permeability                                              Eq        .                                  ⁢        3            
Inductance of an inductor can be defined by mutual inductance and self inductance. On an inductor coil, self inductance is unaffected by skin effect at very low frequencies, therefore, only mutual inductance will be discussed hereinafter. Referring to FIG. 1, two coils S1 and S2 with electric currents are mutually inducted creating an inductance which can be derived from the Neumann formula for mutual inductance as indicted by Eq. 3. Thus, inductance can be improved by reducing the interval R between the two coils S1 and S2 or enlargement of the area of each of the coils S1 and S2.
Moreover, large area layout of the transmission lines can result in high equivalent impedance such that the quality factor of the inductor with high inductance is hindered, which is indicated by Eq. 4:
                    Q        =                              2            ⁢            π            ×            The            ⁢                                                  ⁢            maximum            ⁢                                                  ⁢            stored            ⁢                                                  ⁢            energy                                The            ⁢                                                  ⁢            energy            ⁢                                                  ⁢            dissipated            ⁢                                                  ⁢            per            ⁢                                                  ⁢            cycle                                              Eq        .                                  ⁢        4            
Increasing equivalent impedance will cause an increase of energy dissipation, thereby deteriorating quality factor of the inductor. The input end and output end of a two-port inductor with a large area layout can cause a distance issue during circuit system layout, thus increasing difficulty. Further, as both the desirability for higher density and smaller area of transmission lines increase, fabrication processes encounter various technical difficulties.
U.S. Pat. No. 5,461,353, the entirety of which is hereby incorporated by reference, discloses a tunable embedded inductor structure. Referring to FIG. 2, a tunable coil 10 is embedded in a multi-layered substrate structure. A transistor 18 is controlled by a control signal from a control line 15 to electrically short two adjacent conductive interconnections 14 and 16, thereby regulating inductance of the coil 10. Metal layers functioning as shielding inductance are disposed on the top and bottom of the multi-layered substrate structure, respectively. The advantageous feature is the capability of turning inductance and having a superb quality factor due to distribution of the electromagnetic field confined within the spiral coil. A large circuit layout area, however, is needed to achieve coils with high inductance. Since the input end and the output end of the coil are separated very far apart, a very large circuit layout area is occupied during fabrication of the two-port inductor device.
Further, U.S. Pat. No. 6,384,706, the entirety of which is hereby incorporated by reference, discloses an inductor structure layout with a plurality of planar spiral coils on different layers of a substrate. Each planar spiral coil is connected to each other through conductive interconnections. Referring to FIG. 3A, an inductor device 20 includes a substrate structure composed of a plurality of dielectric layers 25. Two planar spiral coils 26a and 26b are disposed in the substrate structure and connected to each other through an interconnection 27 to improve inductance. The cross section of the inductor device 20 is shown in FIG. 3B. The substrate structure further includes a power source line 24, a ground line 23, and signal lines 22 all of which are connected by contact lines 31 and controlled by integrated circuits 32a, 32b and capacitors 33a, 33b. The abovementioned large inductance coil structure has a deteriorated quality factor due to being prone to electromagnet radiation. Since the input end and the output end of the coil are not disposed on the same layer, which is detrimental to circuit layout, additional conductive lines or interconnections are required to close the input and output ends.
U.S. Pat. No. 6,847,282, the entirety of which is hereby incorporated by reference, discloses a circuit layout with transmission lines disposed on a multi-layered substrate. The transmission lines on each substrate layer are connected through through-holes, blind-holes, or buried-holes, thereby completing a stereographic inductor structure. Referring to FIGS. 4A and 4B, multiple spiral coils 51, 52, 54, and 56 are separately disposed on surfaces 53, 55, 57, and 59 of the laminated dielectric substrate. Each of the multiple spiral coils are connected through conductive interconnections 62, 64, and 66. A patterned shield on the bottom surface of the laminated dielectric substrate serving as ground can effectively block inductance interference. Such an inductor device structure can reduce circuit layout area and maintain high inductance and quality factor. The input end and the output end of the coil are not disposed on the same layer, which is detrimental to circuit layout. Thus, additional conductive lines or interconnections 67 are required to close the input and output ends.